Experience in testing operating filters on a wide variety of paper making pulps has led to a number of conclusions. The manner in which pulp fibers form a sheet and the manner in which such a pulp sheet retains fine particulate matter is dependent upon the type of fiber and the mechanisms by which it is separated from the carrier liquid. During the filtration cycle the fibers retained on a wire mesh form a fibrous mat and the clarity of the filtrate improves with time as the fiber interstices fill up with more and more fine material.
Different pulps require different conditions to form a pulp mat as well as dry a pulp mat and also to optimize the clarity of the "clear" portion of the filtrate.
In the pulp and paper industry, disc and other rotary type filters are used for two primary purposes:
1. To thicken pulps by removing liquid under a combination of gravity and vacuum induced drainage.
2. To act as save-alls and recover separated liquid into filtrate streams containing little particulate matter and streams containing more particulate matter.
For the best disc filter operation a slow draining pulp requires a relatively short time for atmospheric initial pulp mat formation, and a long "vacuum on" cycle to finish forming the pulp mat and to dry the mat as much as possible. A fast draining pulp requires a relatively short "vacuum on" time cycle. The atmospheric mat formation can also be short but a lengthened atmospheric cycle time will give benefit in improved pulp mat formation and disc filter operation.
Even with prior pulp testing it has in the past been impossible to predict and design exactly the optimum disc filter "atmospheric/vacuum on timing" for a given pulp. In addition, some operators must vary their pulp characteristics from day to day depending on the products they wish to manufacture. A single fixed design valve cannot perform satisfactorily under a wide range of feed conditions. The adjustable valve described in the attached disclosure is designed to allow adjustment for a wide variation in the length of "atmospheric on" and "vacuum on" cycles while the filter is operating. In addition, an extra atmospheric leg has been added to the normal filter operation to enable separation of atmospheric filtrate into super clean and atmospheric clean filtrate portions.